1. Field of the Invention
The present invention relates to the chemical-mechanical polishing of semiconductor wafers.
2. Description of Related Art
Polishing generally consists of the controlled abrasion of an initially rough surface in order to produce a specular finished surface free from fracture, scratches, and other defects and of a smoothness approaching the atomic level. This is commonly accomplished by rubbing a pad against the surface of the article to be polished (the workpiece) in a rotary motion in combination with a solution containing a suspension of fine submicron particles (the slurry). Commonly employed pads are made from felted wool, urethane-impregnated felted polyester, or various types of polyurethane plastic.
The polishing rate for such a system is determined by the pressures and velocities employed as well as the concentration of slurry particles in contact with the workpiece at any given time. In order to ensure high and uniform polishing rates, polishing pads are commonly textured to improve slurry flow across the workpiece surface. In addition, the reduction in the contact surface area effected by patterning provides higher contact pressures during polishing, further enhancing the polishing rate. All prior art polishing pads known to the inventors require the simultaneous use of a particle-containing slurry to achieve a detectably high polishing rate; the pad used by itself produces no significant removal or smoothing even when a particle-free liquid is used.
While polishing slurries are universally employed, it is also recognized that their use gives rise to significant problems. First, the particles themselves represent a serious source of contamination when polishing is employed on a semiconductor wafer or device. The use of polishing processes in a clean room seems paradoxical, but is currently widely practiced. The presence of these particles in the clean room facility represents the largest single particulate contamination source in that environment.
Second, the quality of the surface produced is highly dependent upon the particle size distribution and composition in the slurry itself. Anomalously large particles, even in extremely small concentrations, are commonly responsible for scratches and other post-polish mechanical defects. These are highly deleterious to the yield of semiconductor devices processed by polishing. For example, the average particle diameter for slurries used to polish semiconductor devices is typically 0.13 microns, while particles 1 micron or larger may cause fracture. At the solids content of these polishing slurries (typically  greater than 12%) it is practically impossible to use filtration to remove the oversize particles due to clogging effects on the filter medium. Thus expensive and time consuming efforts have been made to control and reduce oversize particles in the slurries employed. However, there are few practical safeguards against their accidental introduction.
A third, and equally significant problem, is the variation in polishing activity over time when slurries are recirculated. A common practice in many industrial applications of polishing is to reuse or recirculate the polishing slurry to reduce manufacturing cost and the quantity of waste products from the operation. Recirculation of cerium oxide based slurries is commonly employed in the optical industry, for example. However, the activity of polishing slurries are commonly observed to vary with time when recirculated. This may be due to the addition of dross, or polishing byproducts from the substrate into the slurry solution, attrition or breakdown of the polishing particles themselves during use, or chemical changes in the particles which reduce activity. The level of variation in recirculated slurries is unacceptably high for processing semiconductor devices. For example, a major application of polishing of semiconductor wafers is the polishing of SiO2 surface films using slurries containing SiO2 particles. Recirculation of this system is exceedingly difficult because the byproducts of the polishing process are coagulated SiO2 particulates derived from in situ polymerization of waste products in the solution. These are practically impossible to distinguish from the original slurry particles, and it is equally impossible to control their size or remove them from the solution. In consequence, the solid particle content of the recirculated slurry continuously increases with time. As the polishing rate is directly proportional to the solids content of the slurry, practical control of the polishing rate is difficult. A serious additional problem is the accidental incorporation of oversize contaminant particles into the recirculating slurry, often due to substrate breakage. The aforementioned difficulties in filtering slurries make it virtually impossible to remove these contaminants.
Because of the above concerns, recirculation of slurry is not practiced in the polishing of most semiconductor devices because of the need to control activity precisely and the avoidance of damage by contaminants. Slurry is simply used once and disposed of as waste. As a result, the cost of slurry and slurry waste disposal is the single largest contributor to the cost of polishing semiconductor devices.
From the above, it is clear that if a polishing process which did not use particulates and which used a fluid which could easily be recirculated and kept in a constant particulate-free state could be developed, it would be extremely attractive for use in the processing of semiconductor wafers.
A wide variety of apparatus for polishing purposes have been disclosed. The most common type, typified by U.S. Pat. Nos. 4,141,180; 4,680,893 and 4,918,870, is comprised of the following features, as illustrated in FIG. 1. The wafer 1 is held by a fixture, or carrier, 2 which is mounted on a rotatable spindle 3. This rotating carrier assembly is pressed against a rotating table 4 on whose upper surface is affixed a polishing pad 5. The simultaneous rotation of carrier and table effects a lateral movement of the pad against the wafer surface. When slurry is fed onto the pad surface 6, the lateral motion in conjunction with the slurry particles effects the polishing action. Most other prior art polishing apparatus designs use the same basic principle, with lateral motion of pad and wafer being effected by several different means including linear motion (see U.S. Pat. No. 5,487,697, Jensen) and ultrasonic vibration (see U.S. Pat. No. 5,245,796, Miller et al.).
All of the prior art polishing apparatus known employ particulate-containing slurries exclusively. None disclose or require for operation liquid delivery systems which include the ability to recirculate, filter, and control the chemical properties of the liquid employed therein as an integral portion of the apparatus, particularly when that liquid is essentially non-particulate. This is not surprising given the aforementioned difficulties of employing recirculation and filtration systems for particulate-containing polishing slurries.
From the above discussion it is evident that the production of a polishing apparatus which can produce uniform polishing action without the use of said slurry particles, is capable of recirculating the liquids used for polishing for extended periods of time while retaining a constant level of polishing activity, and has the means to continuously remove foreign particulate contaminants and waste products from the polishing process would be a highly desirable advancement of the polishing art, and dramatically reduce the cost of polishing of semiconductor devices.
There are other advantages that can be achieved with polishing systems using particle-free solutions. One is to change the fluid chemistry to go from one step of polishing to another in a polishing sequence. This is nearly the same as changing slurries in a standard system, except that the abrasive particles held in the pad remain the same. For example, in a standard system, in which the particles are suspended in a slurry, any addition of salts to increase the ionic strength of the solution will lead to increased settling of the suspended particles. Indeed, this increased settling with increased ionic strength is why some slurries are made as two components which are combined when put into a distribution system where the combined slurry will be subject to continuous mixing to prevent particle settling. The availability of increased salt concentration in a fixed abrasive polishing system gives great flexibility. For example, buffering may be improved to control pH more tightly, in some cases resulting in a better controlled polishing process.
Another mechanism that can be used to advantage in a fixed abrasive polishing system is the effect on the double layer around an abrasive particle. For example, if an increased salt content were added to the polishing solution, there will be a collapse or reduction of the double layer thickness on the abrasive particle. This will result in less repulsive force against another surface and, therefore, increased likelihood of increased polishing abrasive/semiconductor wafer surface interaction. Thus, by modification of the ionic strength of the polishing liquid the polishing rate may be increased or decreased during the various stages of the polishing process merely by changing the ionic strength of the solution.
Another aspect of varying the ionic strength would be in a system comprising more than one abrasive type. If the addition of salts and polishing at various pH levels affects the relative polishing activity of different abrasives, then the ability of each abrasive may be turned on and off in a process with a single fixed abrasive pad. Thus, if one polishing step needs abrasive A and a second step needs abrasive B, the addition of salts to modulate their double layers can be used to enhance or suppress the polishing effect of individual abrasives. The effect of using two different slurries with different abrasives in a standard polishing process can be achieved by varying ionic strength of the particle-free solution when used with a pad comprising different types of abrasives.
Another aspect of this invention is a method of polishing a surface of a semiconductor wafer comprising the steps of: providing a polishing pad having a polishing surface comprising a multiplicity of one type of abrasive particle in said polishing surface; holding said wafer in a carrier such that said wafer surface is in contact with said polishing surface; moving said carrier and/or said polishing surface to provide both pressure on said wafer surface and relative lateral motion between said wafer surface and said polishing surface; and providing a reactive liquid solution essentially free from particulate matter at an interface between said wafer surface and said polishing surface, wherein a change in polishing conditions is effected by changing the ionic strength of said reactive liquid solution.
The preferred type of abrasive particle for use in this invention is a metal oxide, such as alumina, chromia, zirconia, silica, iron oxide, ceria, magnesium oxide, and titania. A further aspect of this invention is the use of two or more types of abrasive particles in the polishing surface.